The preparation of printed circuits, including multilayer printed circuits is known. Powdered materials such as particulate metals have been applied to surfaces having imagewise tacky and nontacky areas by a number of toning methods to produce printed circuit patterns. Representative methods are disclosed in Peiffer U.S. Pat. Nos. 4,054,479, 4,054,483 and 4,157,407. After the particulate metal is applied to the tacky image areas and unwanted particles are removed, e.g., mechanically, from the nontacky image areas, the circuit is formed by one of several additive techniques including fusion of metal particles, electroless plating, etc. The printed circuits formed by these additive processes are useful, but the processes have certain disadvantages.
Peiffer U.S. Pat. No. 4,157,407 is an improvement over the earlier Peiffer U.S. Pat. Nos. 4,054,479 and 4,054,483 wherein conductive interconnections between layers are produced by predrilling holes and registering the holes with the printed circuit patterns. Such predrilling procedures are limited by inherent inaccuracies of registration to printed circuit patterns where circuit lines are not closely spaced. Peiffer U.S. Pat. No. 4,157,407 describes a process for preparing printed circuits wherein a single layer of a photoadhesive, e.g., photopolymerizable composition, is applied to a substrate bearing an electrically conductive circuit pattern. The photoadhesive layer in order to be suitable for the preparation of a printed circuit must be capable of toning in tacky areas without toning in the background areas. The photoadhesive layer also must possess good development latitude with a solvent therefor, adhere metal powder on its vias, and maintain adherence to its substrate through all the steps of preparing a circuit including solvent development and plating. It is difficult to formulate an element having a single photoadhesive layer which provides good properties under all conditions in preparing the printed circuit.
A method of making printed circuits utilizes two exposures of a photopolymerizable element, the first exposure, with a protective film present, defines a circuit pattern; the second exposure, without a protective film present, defines vias. After the first exposure metal powder, e.g., copper, is applied to the circuit pattern. During the second exposure oxygen inhibition is overcome by a gaseous purge, e.g., nitrogen. The amount of metal powder, uniformity of application, and particle size can affect the via exposure level because the exposure wherein the vias are formed takes place after application of the metal powder. If there is insufficient or an uneven gaseous purge during the second exposure, the vias may be poorly reproduced or a loss of metal powder may occur during liquid development.
There is a need for a process for preparing multilayer printed circuits which are insensitive to the exposure environment and properties of the finely divided material, and in their preparation do not undergo loss of the finely divided material due to development.